1. Field of the Invention
The present invention relates to an apparatus and a method for retransmitting data by in a mobile communication system.
2. Description of the Related Art
With the general development of communication technology, mobile communication systems are evolving into communication systems capable of performing high speed data transmission. With the development of the mobile communication systems, various schemes, such as Hybrid Automatic Retransmission reQuest (HARQ) schemes, Adaptive Modulation and Coding (AMC) schemes, etc., have been proposed in order to enable high speed data transmission to be performed and increase the efficiency of resources.
Hereinafter, the HARQ scheme, specifically, an n-channel Stop And Wait (SAW) HARQ scheme, will be described. The n-channel SAW HARQ scheme introduces the following two designs in order to improve the efficiency of a conventional SAW ARQ scheme.
The first scheme is a soft combining scheme. The soft combining scheme is a scheme in which a receiver-side temporarily stores in a soft buffer data having an error, and combines the stored data with the retransmission portion of corresponding data, thereby reducing error occurrence probability. The soft combining scheme includes two schemes, i.e., a Chase Combining (CC) scheme and an Incremental Redundancy (IR) scheme.
When the CC scheme is used, a transmitter-side uses data of the same format in initial transmission and retransmission. That is, in a case in which the CC scheme is used, if m symbols have been transmitted to one coded block in initial transmission, the same number of m symbols are transmitted to one coded block in retransmission as well. The coded block represents user data transmitted during one Transmit Time Interval (TTI). When the CC scheme is used, the same coding rate is applied to the initial transmission and retransmission. Accordingly, the receiver-side combines the initially transmitted coded block with the retransmitted coded block, performs a Cyclic Redundancy Check (CRC) operation by means of the combined coded block, and confirms if an error has occurred according to the results obtained by performing the CRC operation.
When the IR scheme is used, the transmitter-side uses data of different formats in the initial transmission and retransmission. For example, when n bits of user data have been generated as m symbols via channel coding, the transmitter-side transmits only some of the m symbols in the initial transmission, and sequentially transmits the other symbols, excluding the symbols transmitted in the initial transmission, in the retransmission. That is, when the IR scheme is used, a coding rate in the initial transmission is different from that in the retransmission. Accordingly, the receiver-side concatenates the retransmitted coded blocks to the rear portion of the initially transmitted coded block so as to form a coded block having a high coding rate, and performs an error correction. When the IR scheme is used, a version number is used in order to distinguish the coded block transmitted in the initial transmission from the coded blocks transmitted in the retransmission. For example, a version number 1 is assigned to the coded block transmitted in the initial transmission, a version number 2 is assigned to the coded block transmitted in the retransmission, and a version number 3 is assigned to the coded block transmitted in the next retransmission. Accordingly, the receiver-side may combine the initially transmitted coded block with the retransmitted coded blocks by using the version number.
The second scheme for improving the efficiency of the n-channel SAW HARQ scheme is an HARQ scheme. In the case of the conventional SAW ARQ scheme, the transmitter-side transmits next data only when receiving Acknowledgement (ACK) information for the previously transmitted data. Because the transmitter-side transmits the next data only after receiving the ACK information for the previous data, a case may occur in which the transmitter-side must wait for the ACK information even though the transmitter-side can currently transmit data. Different from the conventional SAW ARQ scheme, the n-channel SAW HARQ scheme is a scheme capable of continuously transmitting multiple data even without receiving the ACK information for the previously transmitted data, thereby improving the efficient use of a radio link. That is, when the n-channel SAW HARQ scheme is used, n logical channels are established between a transmitter and a receiver, and the n logical channels are distinguished by specific time or expressed channel numbers, so that the receiver can determine which data a packet received in a random point in time belongs. Accordingly, the receiver can perform a necessary operation, e.g., the receiver can reconstruct data according to a sequence in which the data must be received, can soft-combine corresponding data, etc.
With the development of mobile communication systems, a structure in which different layers may perform functions performed by each layer has started to be introduced in a protocol stack for the improvement of the optimization and throughput. That is, a Medium Access Control (MAC) layer has generally performed the data retransmission function according to the ARQ scheme. Currently, a Physical (PHY) layer has also performed the data retransmission function according to the HARQ scheme. In other words, different layers have independently performed nearly identical functions.
FIG. 1 is a diagram illustrating a general data retransmission operation according to the ARQ scheme and the HARQ scheme in a mobile communication system.
Referring to FIG. 1, a protocol stack of a transmitter has a structure of a PHY layer 110 and an MAC layer 120. Likewise, a protocol stack of a receiver also has a structure of a PHY layer 130 and an MAC layer 140. FIG. 1 illustrates only the PHY layers and the MAC layers in the protocol stack structures of the transmitter and the receiver in order to describe the ARQ scheme and the HARQ scheme. However, it is apparent to those skilled in the art that other layers may exist in addition to the PHY layer and the MAC layer. The data retransmission function according to the HARQ scheme is performed between the PHY layer 110 of the transmitter and the PHY layer 130 of the receiver. The data retransmission function according to the ARQ scheme is performed between the MAC layer 120 of the transmitter and the MAC layer 140 of the receiver.
When an error occurs in the data (i.e., MAC Protocol Data Unit (MPDU)) transmitted to the MAC layer 140 of the receiver from the MAC layer 120 of the transmitter, the MAC layer 140 of the receiver transmits a retransmission request (i.e., Non-Acknowledgement (NACK) information) of the MPDU, which has been transmitted from the MAC layer 120 of the transmitter, to the MAC layer 120 of the transmitter. Then, the MAC layer 120 of the transmitter retransmits the MPDU for which the retransmission has been requested to the MAC layer 140 of the receiver. The NACK information may be defined as a primitive form.
Further, when an error occurs in data (i.e., Physical Protocol Data Unit (PPDU)) transmitted to the PHY layer 130 of the receiver from the PHY layer 110 of the transmitter, the PHY layer 130 of the receiver transmits a retransmission request (i.e., NACK information) of the PPDU, which has been transmitted from the PHY layer 110 of the transmitter, to the PHY layer 110 of the transmitter. Then, the PHY layer 110 of the transmitter retransmits the PPDU for which the retransmission has been requested to the PHY layer 130 of the receiver.
As described in FIG. 1, the MAC layer and the PHY layer independently perform the data retransmission functions according to the ARQ scheme and the HARQ scheme, respectively. That is, because the MAC layer performs the retransmission function according to the ARQ scheme for actually the same data, it is necessary to continuously transmit/receive a primitive representing the ACK information and the NACK information. Therefore, the signaling load may increase. Further, when the primitive is transmitted/received, delay time in the signaling may occur. Therefore, the entire performance of the mobile communication system may deteriorate. In addition, the PHY layer also performs the retransmission function according to the HARQ scheme for the same data, so that redundant transmission of unnecessary data may occur.
Accordingly, it is necessary to provide a new data retransmission scheme capable of not only minimizing the signaling load and the delay time, but also preventing redundant transmission of unnecessary data.